The present invention relates to a brake master cylinder device for use in vehicles such as automobiles, and, more particularly, relates to a brake master cylinder device in which the mechanical advantage obtained by the operator of the vehicle, as he presses on the brake pedal thereof in order to apply actuating force to the braking system of the vehicle, is advantageously arranged to vary.
In many vehicles such as automobiles there are conventionally used hydraulic fluid braking systems in which: a brake pedal or other braking member of the vehicle is depressed by the operator thereof, the force exerted on this braking member is converted by a brake master cylinder device into hydraulic brake fluid pressure, and this hydraulic brake fluid pressure is transmitted via a conduit system to one or more fluid actuators of braking systems, which act upon the road wheels of the vehicle, and in which a friction member such as a brake shoe or a brake pad is pushed by said hydraulic brake fluid pressure against a rotating member which rotates along with the road wheels of the vehicle such as a brake drum or a brake disk, in order to perform braking action.
In such a hydraulic fluid braking system, typically, there is provided a certain clearance between the friction member such as a brake shoe or a brake pad, and the rotary member such as a brake drum or brake disk. Recently, it has become practiced to provide a considerably large clearance between such a friction member and such a rotary member, from the point of view of guarding against dragging or rubbing of the brakes of the vehicle, which can greatly reduce the efficiency of operation of the vehicle, and increase the fuel consumption thereof.
In the process of application of such a braking system, there are two distinct phases.
First, as the brake pedal or other braking member of the vehicle is initially moved by the operator thereof, the brake master cylinder device expels fluid into the conduit system and thence into the abovementioned hydraulic actuator or actuators, so as, initially, to take up the said clearance, by moving the friction member or members such as a brake shoe or brake pad into initial gentle contact with the rotary member or members such as a brake drum or brake disk. The hydraulic brake fluid pressure required for taking up this initial clearance is relatively low. However, the total amount of hydraulic brake fluid required to be displaced through the conduit system, for taking up this clearance, is quite large, since, as explained above, the clearance may be quite large. Therefore, in order to provide this fairly large quantity of hydraulic brake fluid through said conduit system at fairly low pressure, without the braking actuating member of the vehicle such as a brake pedal requiring to be moved through an undesirably large distance, it is desirable that the diameter of the hydraulic piston of the brake master cylinder device should be quite large, in order that, per unit amount of movement of this hydraulic piston, a comparatively large quantity of hydraulic brake fluid should be expelled from the brake master cylinder device through the conduit system. With such a large diameter brake master cylinder device, the mechanical advantage provided for the operator of the vehicle, as he presses upon the braking member thereof, is not very high.
On the other hand, in the second phase of braking operation, when the abovementioned clearance has been taken up, so that the friction member such as a brake pad or brake shoe is in gentle contact with the rotary member such as a brake disk or a brake drum, then, in order actually to perform substantial braking action to retard the motion of the vehicle, the pressing force between this friction member and this rotary member needs to be substantially increased. As this pressing force is increased, the friction member does not move very far, as compared to its amount of motion while taking up the abovementioned clearance in the first phase of operation. However, the force required to move the friction member is much greater than was required in the abovementioned first phase of operation. Thus, in this second phase of operation, a not very large amount of hydraulic brake fluid needs to be provided through the conduit system leading to the hydraulic actuator, but this not very large supply needs to be made available at quite high hydraulic pressure. Accordingly, therefore, it is desirable, for this phase of operation, to employ a brake master cylinder device, the diameter of the hydraulic piston of which is quite small, so that a high mechanical advantage is obtained by the driver of the vehicle as he presses on the braking member thereof, so that a sufficiently high hydraulic brake fluid pressure may be provided to the conduit system and to the hydraulic actuator, without an undue amount of pressure needing to be exerted by the driver on the brake member. In other words, the mechanical advantage provided by the brake master cylinder device should, in this phase, desirably be at a high level, which entails a small diameter for the hydraulic piston of the master cylinder device.
Therefore, it is seen that the requirements with regard to the diameter of a piston of a brake master cylinder device, in the abovementioned first and second stages of operation of the braking system of the vehicle, are somewhat opposed. Accordingly, therefore, in conventional designs of brake master cylinder device, a compromise has been found between the requirement of the first phase of brake operation, which is for a large diameter for the piston of the brake master cylinder device, and the requirement of the second phase, which is for a small diameter of the piston of the brake master cylinder device. However, finding this compromise has become more difficult, according to recent developments as mentioned above, in which the initial clearance between a friction member such as a brake pad or brake shoe, and a rotary member such as a brake disk or brake drum, has been increased. This has presented a substantial difficulty with regard to current design of brake master cylinder devices.